How Fluid Moves Between Blood and Tissues

Every time blood passes through the smallest vessels in your body, the capillaries, it delivers fluid directly to your tissues, and reclaims most of it on the way out. This lesson looks at that exchange, and at the small leftover it cannot reclaim, which is where the next lesson picks up.

Blood does more than carry cells and nutrients around the body. Every time it passes through the smallest vessels in your body, the capillaries, it delivers fluid, nutrients and proteins directly to your tissues. It also picks some of that fluid back up on the way out. This lesson looks at that exchange in detail. Blood is mostly a fluid exchange service Picture a capillary as a hosepipe with a slightly leaky wall. At one end, blood arrives under real pressure, pushed along by the heart. That pressure squeezes a small amount of watery fluid out through the vessel wall and into the space between your cells. This fluid bathes every cell directly, delivering oxygen and nutrients at the point of use. By the time blood reaches the far end of the same capillary, the pressure pushing fluid out has dropped. Meanwhile, large proteins left behind in the blood's plasma, mainly albumin, pull water back in, like a sponge drawing fluid toward itself. Most, but not quite all, of the fluid that leaked out gets pulled back in. The leftover fluid needs somewhere to go That small leftover amount, several litres a day across your whole body, does not just accumulate in your tissues. It drains into an entirely separate network of vessels: the lymphatic system, which the next lesson covers in full. For now, the key point is this: blood and lymph are two connected fluid systems, not one. Blood is pressurised and pumped. Lymph is unpressurised and drains passively, one way, back toward the blood. What a doctor checks when this system is affected A doctor cannot see fluid balance directly, but one simple check reveals a great deal. Looking and Feeling: Pressing gently on the skin over a bone, such as the shin, can reveal whether excess fluid has built up in the tissues. If an indent stays behind, this sign is called pitting oedema. Special tests: Blood tests measuring protein levels can help doctors understand fluid exchange balance. Checks of the heart, kidneys and liver are also useful when swelling is significant or persistent. When this system is affected When fluid leaks out of the capillaries faster than it can be reclaimed or drained, people may notice swelling in the ankles or legs, called oedema. Swelling is explored further in a future lesson.

Blood constantly exchanges fluid with tissues at the capillaries: pressure pushes fluid out early in each capillary, and proteins left behind in the blood pull most of it back in by the far end, with the small remainder draining away through a separate system.